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8cfcdee95e
Floating point conversions inside vectorized loops have performance implications but are very subtle. The user could specify a floating point constant, or call a function without realizing that it will force a change in the vector width. An example of this behaviour is seen in https://godbolt.org/z/M3nT6c . The vectorizer should indicate when this happens becuase it is most likely unintended behaviour. This patch adds a simple check for this behaviour by following floating point stores in the original loop and checking if a floating point conversion operation occurs. Reviewed By: fhahn Differential Revision: https://reviews.llvm.org/D95539
70 lines
3.4 KiB
LLVM
70 lines
3.4 KiB
LLVM
; RUN: opt -force-vector-interleave=2 -force-vector-width=4 -loop-vectorize -pass-remarks-analysis=loop-vectorize -disable-output < %s 2>&1 | FileCheck %s
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; CHECK: remark: mixed-precision.c:3:26: floating point conversion changes vector width. Mixed floating point precision requires an up/down cast that will negatively impact performance.
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define void @f(float* noalias nocapture %X, i64 %N) {
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entry:
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br label %for.body
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for.cond.cleanup:
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ret void
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for.body:
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%i = phi i64 [ %inc, %for.body ], [ 0, %entry ]
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%arrayidx = getelementptr inbounds float, float* %X, i64 %i
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%0 = load float, float* %arrayidx, align 4
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%conv = fpext float %0 to double, !dbg !9
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%mul = fmul double %conv, 0x3FD5555555555555
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%conv3 = fptrunc double %mul to float
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store float %conv3, float* %arrayidx, align 4
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%inc = add nuw i64 %i, 1
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%exitcond.not = icmp eq i64 %inc, %N
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br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
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}
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; CHECK: remark: mixed-precision.c:8:8: floating point conversion changes vector width. Mixed floating point precision requires an up/down cast that will negatively impact performance.
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; CHECK: remark: mixed-precision.c:7:16: floating point conversion changes vector width. Mixed floating point precision requires an up/down cast that will negatively impact performance.
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; CHECK-NOT: remark: mixed-precision.c:7:16: floating point conversion changes vector width. Mixed floating point precision requires an up/down cast that will negatively impact performance.
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define void @g(float* noalias nocapture %X, float* noalias nocapture %Y, i64 %N) {
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entry:
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%pi = alloca double
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store double 0x400921FB54442D18, double* %pi
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%fac = load double, double* %pi
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br label %for.body
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for.body:
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%i = phi i64 [ %inc, %for.body ], [ 0, %entry ]
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%arrayidx = getelementptr inbounds float, float* %X, i64 %i
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%0 = load float, float* %arrayidx, align 4
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%conv = fpext float %0 to double, !dbg !10
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%mul = fmul double %conv, %fac
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%conv1 = fptrunc double %mul to float
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store float %conv1, float* %arrayidx, align 4
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%arrayidx5 = getelementptr inbounds float, float* %Y, i64 %i
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%1 = load float, float* %arrayidx5, align 4
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%conv2 = fpext float %1 to double, !dbg !11
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%mul2 = fmul double %conv2, %fac
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%conv3 = fptrunc double %mul2 to float
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store float %conv3, float* %arrayidx5, align 4
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%inc = add nuw nsw i64 %i, 1
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%exitcond.not = icmp eq i64 %inc, %N
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br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
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for.cond.cleanup:
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ret void
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}
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!llvm.dbg.cu = !{!0}
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!llvm.module.flags = !{!3, !4}
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!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 12.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
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!1 = !DIFile(filename: "mixed-precision.c", directory: "/tmp/mixed-precision.c")
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!2 = !{}
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!3 = !{i32 2, !"Debug Info Version", i32 3}
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!4 = !{i32 1, !"wchar_size", i32 4}
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!6 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
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!7 = distinct !DISubprogram(name: "g", scope: !1, file: !1, line: 5, type: !8, scopeLine: 5, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
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!8 = !DISubroutineType(types: !2)
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!9 = !DILocation(line: 3, column: 26, scope: !6)
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!10 = !DILocation(line: 7, column: 16, scope: !7)
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!11 = !DILocation(line: 8, column: 8, scope: !7)
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